Myotonic dystrophy type 1 (DM1) is a neuromuscular disease caused by an abnormal CTG repeat expansion in the 3’UTR region of DMPK gene. In patients, the CTG repeat size varies from 50 to thousands CTG and usually increases across generations and over time in tissues. Larger expansions are associated with more severe symptoms and a decreasing age of onset. It is our hypothesis that the development of innovative therapeutic strategies, aimed at decreasing the CTG repeat length, and thus to stop or reverse the progression of the disease, may improve the quality of life of DM1 patients. The specific aim of our project is to uncover bioactive molecules able to induce repeat contractions taking advantage of a chromosomal GFP reporter assay that can accurately measure both expansions and contractions in the same population of HEK293 cells. We performed a large-scale screen for pharmacologically relevant chemical modulators of instability using the Prestwick Library (>1200 FDA-approved drugs) in different HEK293 cell models of CTG.CAG repeat instability. We have identified several candidate molecules involved in DNA repair and chromatinization that may change the size of the CTG repeat tract. Using the latest long-read sequencing technology developed by Pacific biosciences, we are currently estimating the direct effect of each molecule on the dynamics of CTG.CAG repeat instability using HEK293 models as well as DM1 fibroblasts. The direct perspective of our work is to identify new small molecules and new druggable targets that promote CAG.CTG repeat contractions, offering new therapeutic perspectives for DM1 but also for other trinucleotide repeat diseases.